Polyimide has been widely used in electronic material peripherals and OA instrument device peripherals due to its excellence in heat resistance, reliability for electric insulation, chemical resistance and mechanical characteristics. For example, it is used for forming an insulation film or a protective coating material on semiconductor devices, a base material or a surface protecting material for flexible circuit board or integrated circuit and an interlayer insulating coating layer or a protective coating layer for fine circuit.
In general, many of polyimides are insoluble in a solution and are lacking in processing ability. Therefore, it is general that polyamic acid (also called polyamide acid) which is a precursor thereof is polymerized in a solution, processed into a film or other molded products and then imidized. Usually, heating at 250° C. or higher is needed for conversion of polyamic acid into an imide.
However, there has been a problem when the processing is done by the above means such as that the electronic material is not durable against the high temperature, adhesiveness to the base material is insufficient, etc. because the heating temperature therefor is high. Moreover, since water is generated when polyamic acid is subjected to ring closure and converted into polyimide, there is another problem such as that shrinking upon curing is resulted whereby the base material is curled.
In addition, polyamic acid has such a characteristic that a decrease in molecular weight and a branching of resin structure are apt to happen due to hydrolysis caused by water generated by imidization and by water existing in the liquid. As a result, there is a tendency that its solution viscosity is apt to become low with elapse of time (cf. Non-Patent Document 1). The instability of the solution viscosity as such is particularly significant during the preservation at room temperature and, since no product having a uniform quality can be produced, that is not favorable in view of its practical application.
Under such circumstances, there has been investigated for a polyimide which is soluble in organic solvents being such a type where imidization is made to finish in an organic solvent. For example, in the Patent Document 1, there is a proposal for a modified polyimide of such a type which is soluble in organic solvents and has a polybutadiene skeleton and there are disclosed examples where a thermosetting composition using a blocked isocyanate as a curing agent for a terminal acid anhydride group-containing polyimide produced by an isocyanate method is used as an overcoating agent for flexible circuit board.
However, this thermosetting composition has the following problems:
(1) Since the acid anhydride group is present only in the terminal of the resin, no sufficient thermosetting property can be achieved;
(2) Since the resin skeleton contains urethane bond, heat resistance is low; and
(3) Intramolecular cross-linking is induced by oxidation which is specific to the polybutadiene skeleton whereupon the resin is gelled during the reaction or the stability as the composition during its preservation significantly lowers.
In recent years, there has been an increasingly severe demand for the characteristics in terms of materials for electronic instruments such as OA instruments and domestic electronic appliances in view of tendency for high density and high function and also for environments. With regard to a flexible printed circuit (FPC) board for example, there has been a demand for characteristics such as flexibility, low warpage, solder dip resistance, migration resistance and plating resistance.
For the surface protection of flexible printed circuit board and also for the prevention of sticking of solder upon mounting thereof, there have been a method where adhesion is conducted using an adhesive after a polyimide film called a coverlay film is punched using a metal mold meeting a pattern and also a method where a liquid or filmy solder resist agent of a thermosetting or an ultraviolet setting type is used. The latter is particularly advantageous in view of workability.
Moreover, as a result of the tendency in recent years that weight and size of electronic instruments are becoming smaller, a flexible printed circuit board is also becoming lighter and thinner whereby the affection of flexibility and shrinking upon curing of a solder resist agent is becoming to appear more significantly. Therefore, it is the current status that a solder resist agent of a curing type now does not satisfy the demanded characteristics in terms of flexibility and shrinking upon curing.
Further, if and when a polymer material (resin material) ignites due to abnormal heating caused by malfunctioning of the parts, there is a risk that it causes a fire whereby the polymer material per se is demanded to have a self-extinguishing property (non-inflammability/flame retarding property). For example, with regard to a solder resist agent used for a flexible printed circuit board, there is a tendency of reducing the environmental load in recent years and there has been a demand for making it lowly harmful, lowly fuming and flame retarding under a non-halogen (halogen-free) state.
As to an art for achieving the flame retarding property, there may be exemplified a method where a flame retardant of a phosphate type is added but, for achieving the flame retarding property of a high degree, it is necessary to add a large amount of a flame retardant whereby there is a risk that the characteristics such as adhesive property, mechanical characteristic and heat resistance become low and there may also cause a problem such as that the flame retardant per se is bled out whereby the adhesive property lowers with elapse of time.
As such, a solder resist agent of a curing type is demanded to satisfy various properties depending upon the use. Particularly when it is used for electronic parts or the like, a flame retarding property is demanded as an important property in addition to solder dip resistance, flexibility, etc. and, when the flame retarding property is low, the use is limited.
In order to cope with the high demand for the peripheral components for electronic materials as such, various investigations have been conducted already but the thing which fully satisfies all characteristics has not been achieved yet. For example, the Patent Document 2 discloses a thermosetting composition in which an epoxy compound is used as a curing agent for a soluble polyimide having a carboxyl group in the side chain. However, since water is generated upon a ring closure reaction of polyamic acid during the polymerization of resin, branching of the resin structure is resulted or molecular weight of the resin hardly rises. Therefore, the cured product is low in its strength and is poor in its durability. When polymerization is carried by this method, it is difficult to achieve high acid value and to fully raise the molecular weight of the resin. Further, although excellent heat resistance and softness are achieved when a silicone compound is copolymerized, adhesiveness to the base material is low and flexibility is weak.
In the Patent Document 3, there is a proposal that a thermosetting resin composition containing a thermosetting polyurethane having high acid value and a phosphorus atom-containing organic filler is used as a solder resist agent or the like for a circuit base material. In the thermosetting polyurethane used here, polymerizing property of a carboxyl group-containing dihydroxy compound which is a starting material is low whereby molecular weight distribution is apt to expand, cross-linking points are unbalancedly present in particular areas in the main chain of the resin and unreacted substance abundantly remains. Therefore, no sufficient thermosetting property and flexibility can be achieved and heat resistance is low due to the residence of the unreacted substance.
In the Patent Document 4, there is disclosed an example where a thermosetting resin composition prepared by compounding polyamideimide with an epoxy compound as a curing agent is used as an overcoating agent for a flexible circuit board. Since this thermosetting resin composition has a cross-linking point only in the resin terminal, no sufficient thermosetting property is achieved. In addition, since the resin skeleton contains a urethane bond, heat resistance is low.
In the Patent Document 5, there is disclosed an example where a thermosetting urethane resin is compounded as an adhesive and a coating agent for electronic material peripherals. Since the thermosetting urethane resin used here is polymerized by a polymerizing method via a urethane polymer, the resin skeleton contains a urethane bond. Thus, heat resistance of the resin is low in spite of excellent thermosetting property and flexibility.
Moreover, a flexible printed circuit board may become under a high temperature condition due to heat generation and it has been demanded that high reliability can be maintained even under such an environment. However, when the changes in the resin dimension at high temperature are big, detachment from metal wiring such as copper forming the circuit happens resulting in short circuit or broken wire. Further, even in a flexible multi-layered board which has been receiving public attention as a thin board in recent years, the same problem also happens when there is a big difference in the dimensional changes upon heating among an adhesive layer which adheres flexible boards each being in a single layer, a polyimide film forming the flexible board and a metal wiring such as copper forming the circuit.
In the Patent Document 6, there is disclosed an art for improving the properties at high temperature by the joint use of an epoxy resin having excellent heat resistance with an inorganic compound. However, the improving effect for solvent resistance cannot be expected.